/Engine/Source/Runtime/GeometryCore/Public/Util/IndexUtil.h
Find third vertex of triangle that is not VertexID1 or VertexID2. Triangle must contain VertexID1 and VertexID2 or result will be incorrect.
VertexID of third vertex, or incorrect result if precondition is not met
namespace IndexUtil { template<typename T, typename Vec> int IndexUtil::FindTriOtherVtxUnsafe ( T VertexID1, T VertexID2, const Vec & TriangleVerts ) }